Evaporative heat exchangers are widely used in many applications where it is necessary to cool or condense fluid and/or gas that must be maintained out of contact with the heat exchange medium to which the heat is transferred. For example, air conditioning systems for large buildings employ evaporative heat exchangers for carrying out a portion of the heat exchange that is essential to the cooling process. In these systems, air inside the building is forced passed coils containing a cooled refrigerant gas thereby transferring heat from inside the building into the refrigerant gas. The warmed refrigerant is then piped outside the building where the excess heat must be removed from the refrigerant so that the refrigerant gas can be re-cooled and the cooling process continued. In addition, industrial processes such as chemical production, metals production, plastics production, food processing, electricity generation, etc., generate heat that must be dissipated and/or disposed of, often by the use evaporative heat exchangers. In all of the foregoing processes and numerous other processes that require the step of dissipating or disposing of heat, evaporative heat exchangers have been employed.
The general principle of the evaporative heat exchange process involves the fluid or gas from which heat is to be extracted flowing through tubes or conduits having an exterior surface that is continuously wetted with an evaporative liquid, usually water. Air is circulated over the wet tubes to promote evaporation of the water and the heat of vaporization necessary for evaporation of the water is supplied from the fluid or gas within the tubes resulting in heat extraction. The portion of the cooling water which is not evaporated is recirculated and losses of fluid due to evaporation are replenished.
Conventional evaporative heat exchangers are presently in widespread use in such areas as factory complexes, chemical processing plants, hospitals, apartment and/or condominium complexes, warehouses and electric generating stations. These heat exchangers usually include an upwardly extending frame structure supporting an array of tubes which form a coil assembly. An air passage is formed by the support structure within which the coil assembly is disposed. A spray section is provided usually above the coil assembly to spray water down over the individual tubes of the coil assembly. A fan is arranged to blow air into the air passage near the bottom thereof and up between the tubes in a counter flow relationship to the downwardly flowing spray water. Heat from the fluid or gas passing through the coil assembly tubes is transferred through the tube walls to the water sprayed over the tubes. As the flowing air contacts the spray water on the tubes, partial evaporation of some of the spray water occurs along with a transfer of heat from the spray water to the air. The air then proceeds to flow out of the heat exchanger system. The remaining unevaporated spray water collects at the bottom of the conduit and is pumped back up and out through the spray section in a recirculatory fashion.
Current practice for improving the above described heat transfer process includes increasing the surface area of the heat exchange tubes. This can be accomplished by increasing the number of coil assembly tubes employed in the evaporative heat exchanger by “packing” the tubes into a tight an array as possible, maximizing the tubular surface available for heat transfer. The tightly packed coils also increase the velocity of the air flowing between adjacent tube segments. The resulting high relative velocity between the air and water promotes evaporation and thereby enhances heat transfer.
Another practice currently employed to increase heat transfer surface area is the use of closely spaced fins which extend outwardly, in a vertical direction from the surface of the tubes. The fins are usually constructed from a heat conductive material, where they function to conduct heat from the tube surface and offer additional surface area for heat exchange.
In addition, another method currently used to increase heat exchange is the use of splash type fill structures placed between individual tubes in a coil assembly that can function to provide additional water surface area for heat transfer.
These current practices can have drawbacks. For example, the use of additional tubes requires additional coil plan area along with increased fan horsepower needed to move the air through the tightly packed coil assembly, increasing unit cost as well as operating cost. In addition, placement of fins between the individual tubes may make the heat exchanger more susceptible to fouling and particle build up. Further, indiscriminate placement of fill sheets within coils assemblies can cause performance degradation by hindering air flow, and the fill sheets can act as an insulator where they abut the tubes, and/or can cause heat already transferred to the air to be transferred back to the cooling water.
Accordingly, it is desirable to provide a method and apparatus for effectuating desirable, evaporative heat exchange that can offer a substantial reduction in parts, improved efficiency and or reduction of complex and costly assembly of components. It is also desirable to provide increased evaporative heat exchange without undesirably increasing the size of the unit, the manufacturing cost of the unit, and/or operating cost of the unit.